1. Field of the Invention
The present invention relates to a toner for dry electrostatic charge image development which is used for developing an electrostatic latent image formed in processes of electrophotography, electrostatic recording, and electrostatic printing, and also relates to an image forming apparatus using such toner, and a process cartridge using such toner.
2. Description of the Related Art
In the field of electrophotography, heretofore, research and development has been carried out using various inventive processes and technological approaches. In electrophotography, a surface of a photoconductor is charged, the charged surface of the photoconductor is exposed to light to form an electrostatic latent image, the resultant electrostatic latent image is developed with a toner having a color to form an toner image, and the resultant toner image is transferred to a medium such as transfer paper, then the transferred toner image is fixed with a heat roller, and the like to form an image.
For a method of fixing a toner, a contact heat fixing method, such as a fixing method using a heat roller, is widely employed. Fixing devices used in the heat roller fixing method are equipped with a heat roller and a pressure roller, and used for thermofusing and fixing toner images onto a recording sheet by feeding the recording sheet, on which the toner image has been transferred, through a pressure welding part (a nip portion) between the heat roller and the pressure roller.
Examples of resins used in toners mainly include vinyl polymerization resins and resins having polyester backbones. Recently, also so-called hybrid resins having both types of backbones of the above two resins, in which the above two types of resins are combined for use, are used, because the above two types of resins have advantages over each other and disadvantages compared to each other with respect to functional characteristics of toners, such as flowability, mobility, electrostatic chargeability, fixing ability, and image characteristics. Examples of a method for producing toners include, in addition to a so-called kneading pulverizing method which has been conventionally used, a production method called a wet granulation method or a chemical toner method, such as a suspension method and an emulsification method which uses an organic solvent with a water-based solvent; a suspension polymerization method in which polymerizable monomer droplets are controlled for polymerization to directly obtain toner particles; and an agglutination method in which fine emulsified particles are prepared and agglutinated to obtain toner particles.
In the above-mentioned contact fusing method, the heating temperature is set as low as possible for saving energy for consumption in the equipment. Therefore, resins for use in toners used in such equipment are preferably resins which melt at low temperatures. In an electrophotographic process, however, there are steps in which mechanical stress or thermal stress is exerted on such toners. Thus, the resins for use in such toners must meet requirements of thermal properties, such as glass transition temperature, for preventing blocking, and the like and requirements of molecular weights for preventing toner cracking. Lowering the heating temperatures and meeting the above two requirements are two desirable but incompatible features, thus it is important to achieve an excellent trade-off between the two features for developing such toners. From the viewpoint of achieving such an excellent trade-off, a known, so-called core/shell type toner has been proposed, which contains a resin having favorable thermal fusing ability inside covered by a resin having favorable preventability of blocking outside, and which is designed to contain polyester resins and styrene-acrylate resins for core materials (see Japanese Patent Application Laid-Open (JP-A) Nos. 2006-285188 and 2007-093809).
Although polyester resins have excellent low-temperature fixing ability, they have poor dispersibility of releasing agents. Furthermore, although styrene-acrylate resins have excellent dispersibility of releasing agents, they don't have good low-temperature fixing ability. Thus at present, it is desired that a hybrid resin, which may achieve favorable dispersibility of releasing agents while achieving favorable low-temperature fixing ability, be used for a core material in the toner. Although, in recent years many researchers have undertaken the work for developing a shell for the core containing the hybrid resin, it is found that it takes a long time period to unify the core and the shell by heating, requiring considerable load which must be applied to the environment of the unification process (JP-A No. 2007-003840).